Pressure ratio control system for a continuously variable drive unit

ABSTRACT

A control for a continuously variable unit (CVU) provides control pressures to the variable sheave portions of the CVU pulleys. A first of the pulleys has a direct control pressure and the second of the pulleys has a control pressure proportional to the direct control pressure. The proportional pressure is established by a pair of control valves. One of the control valves issues a bias pressure, proportional to the commanded ratio in the CVU, to the other of the control valves. The other control valve communicates a sheave control pressure, proportional to or equal to the bias pressure, to the second pulley to control the relative position of the variable sheave.

TECHNICAL FIELD

This invention relates to control systems for continuously variablepower transmissions and more particularly to pressure control systemsfor controlling the pressure ratio on the drive sheaves.

BACKGROUND OF THE INVENTION

Many continuously variable drive units (CVU) utilize a pair of spacedadjustable sheaves or pulleys that are drivingly connected by a flexibletorque transmitter such as a belt. In most modern CVUs, the belt ismetal member comprised of a plurality of metal plates maintained in acontinuous loop by a plurality of thin metal bands. These metal beltspermit a large amount of torque transfer as compared with conventionalrubber V-belts.

The diameters about which the belt is trained on the sheaves determinesthe transmission ratio between the input and output shafts connectedtherewith. For example, if the diameters are equal, a one to one ratiois present, if the input sheave diameter is smaller than the outputsheave diameter, an underdrive ratio is provided and if the input sheavediameter is greater than the output sheave diameter, an overdrive ratiois presented. During operation, the sheave diameters are controlled toprovide a continuously variable ratio as opposed to a step ratio commonto most power transmissions. The ratio between the input and outputshafts is varied from an underdrive at vehicle launch to an overdriveratio at cruising speeds.

The ratio between the shafts may be held at any of the available ratiosduring vehicle operation. The ratio between the sheaves is generallymaintained by hydraulic pressure applied to a moveable sheave in each ofthe pulleys. The pressure is supplied by a control system whichmaintains a high pressure on one of the pulleys, usually the outputpulley, and varies the pressure on the other pulley. At least one priorart control incorporates a position feedback indicator or valve tocontrol the pressure in the input pulley. An electronic control issues aratio command and the pressure at the input pulley is varied relative tothe output pulley until the commanded ratio is present. The feedbackindicator informs the control when the commanded ratio is achieved.

Other prior art control use direct pressure control to establish theratio between the input and output pulleys. The output pulley ispressurized at a level determined by the torque load and the inputpulley control pressure is varied in accordance with a look-up tablewhich stores the pressure values for each ratio. The control issues acommand to adjust the pressure to provide the calculated pressure forthe requested ratio.

While both of these systems will provide a ratio control in a CVU, theyboth have some disadvantages as to cost and complexity. The positionfeedback controls require mechanical elements disposed in abutment withthe input pulley. These systems also require a mechanical connectionbetween the feedback member and a valve member disposed to control thepressure on the pulley. The direct pressure systems use an electronicprocessing unit and need sufficient memory space for the tables used bya digital computer for controlling the CVU and software to provide thelookup function. Also the currently available direct pressure controlsystems do not make non electrical accommodations for fluctuatingpressure in the output pulley control pressure which can occur duringvehicle operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved ratiocontrol for a continuously variable drive unit.

In one aspect of the present invention, a ratio control used a directpressure control wherein the input or primary pulley pressure isdirectly related to the output or secondary pulley pressure. In anotheraspect of the present invention, the primary pulley control pressure isa ratio of the secondary pulley pressure.

In yet another aspect of the present invention, the control pressure atthe primary pulley is determined by a pulse width modulated valve havingthe input pressure thereto equal in value to the pressure at thesecondary pulley. In a further aspect of the present invention, theoutput pressure from the pulse width modulated valve is a percentage ofthe input pressure and is therefore a ratio of the input pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a powertrain having acontinuously variable unit and an electro-hydraulic control systemincorporating one embodiment of the present invention for controllingthe ratio in the continuously variable unit.

FIG. 2 is a schematic representation of a powertrain having acontinuously variable unit and an electro-hydraulic control systemincorporating another embodiment of the present invention forcontrolling the ratio in the continuously variable unit.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

A powertrain 10 is comprised of an engine 12, a forward/reverse gear set14 a continuously variable unit (CVU) 16 and a differential mechanism18. This type of powertrain is well known. The engine 12 is aconventional internal combustion engine which provide power over a rangeof speeds. The gear set 14 is preferably a planetary gearing whichprovides forward and reverse input to the CVU 16. As is well known, thegear set 14 also includes conventional fluid operated friction devicesto control the ratio in the planetary gearing.

The CVU 16 has a variable primary or input pulley 20 and a variablesecondary or output pulley 22. The primary pulley 20 is drivinglyconnected with the gear set 14 and the secondary pulley 22 is drivinglyconnected with the differential 18. The pulleys 20, 22 are drivinglyinterconnected by a flexible transmitter or belt 24. The belt 24 ispreferably a metal push type belt. The belt 24 transmits power from theprimary pulley 20 to the secondary pulley 22.

The pulley 20 has a variable sheave 26 which is hydraulically positionedrelative to a fixed sheave 28 by hydraulic pressure in a passage 30. Thepulley 24 has a variable sheave 32 which is hydraulically positionedrelative to a fixed sheave 34 by hydraulic pressure in a passage 36. Thesheaves 26 and 32 each have a control chamber with the control chamberof the sheave 26 being twice as large as the control chamber of thesheave 32. The structure of variable sheaves and the pressure chambersthereof are well known to those skilled in the art of CVU construction.The CVU 16 and gear set 14 may be constructed in accordance with many ofthe well-known variable ratio transmissions such as those described inU.S. Pat. No. 4,644,820 issued to Macey et al on Feb. 24, 1987 and U.S.Pat. Nos. 5,803,858 and 5,803,859 both issued to Haka on Sep. 8, 1998and assigned to the assignee of this application. The pressure at thepulleys 20, 22 positions the belt 24 to thereby establish thetransmission ratio between the pulleys 20, 22. The ratio between thepulleys 20, 22 is continuously variable through a predetermined range byadjusting the respective drive diameters of the pulleys 20, 22 with thehydraulic pressure.

The passages 30 and 36 are components in an electro-hydraulic controlsystem generally designated 38. The control system 38 has a positivedisplacement pump and electro-hydraulic control 40. The control 40 caninclude a conventional central processing unit and preprogrammed digitalcomputer, not shown, which controls a plurality of valve mechanisms, notshown. The valve mechanisms provide maximum system pressure and pressureto the gear set 14 through passages 42 and 44. The control system 38determines which gear ratio will be selected in the gear set 14 and thepressure level in the passage 36.

The passage 36 is also in fluid communication with a pulse widthmodulated (pwm) control valve 46 and pressure control valve 48. The pwmvalve 46 has a pwm control solenoid 50 which receives electrical signalsfrom the control 40 through a wire 52. The electrical signals provide aduty cycle to the pwm solenoid 50 which determines the output pressureof the valve 46 which is distributed to a control pressure passage 54.As is well known with pwm solenoids, a control pressure proportional tothe duty cycle is provided. The duty cycle operates between zero and onehundred (100%) percent. Thus, the output pressure is a percentage of theinput pressure. In this instance, the pressure is passage 54 is apercentage of the pressure in passage 36 which is equal to the controlpressure at the secondary pulley 22.

The pressure control valve 48 is a pressure regulating valve whichreceives input pressure via passage 36 and delivers a regulated pressureto passage 30. The valve 48 has a bias spring 55 which urges the valveto an open position to ensure a minimum pressure is always present inthe passage 30. The pressure in passage 30 is communicated with thevalve 48 to urge the valve toward a closed or reduced pressurecondition. Thus, the output pressure the valve 48 is regulated by adownstream pressure.

The bias pressure, in passage 54, is created by the valve 46 asdescribed above. The pressure in passage 54, which is proportional tothe pressure in passage 36, is communicated to the valve 48 to urge thevalve 48 to an open or higher pressure position. The bias pressure onthe valve 48 is proportional to the input pressure in passage 36 suchthat the output pressure of the valve 48 in passage 30 is proportionalto the pressure in passage 36. The control pressure at the pulley 22 isequal to the pressure in the passage 36 and the control pressure at thepulley 20 is equal to the pressure in the passage 30.

Since the pressures in the passages 36 and 30 are proportional, thesqueezing forces on the pulleys 22 and 20 are proportional. The driveratio between the pulleys 20 and 22 is established by the controlpressures and therefore, with the present invention, the ratio betweenthe pulleys 20 and 22 is controlled by the valves 50 and 48. Thepressure in passage 36 is determined by the control 40. The control 40will establish the pressure in the passage 36 is accordance with vehicleand engine operating parameters. The control 40 uses data includingengine speed, input torque and vehicle speed, to name a few, toestablish the pressure in the passage 36. Any changes in the pressure inthe passage 36 will be immediately reflected in the pressure in thepassage 30 such that the desired ratio will be maintained in the CVU.

The passage 30 is also in fluid communication with a pressure limitvalve 56. The valve 56 will prevent the variable sheave 26 of the pulley20 from being subjected to unusually high pressure. The pressure atwhich the valve 56 will provide pressure relief is beyond the normalrange of control pressures for the sheave 26.

As discussed above, the pressure level at the pulley 22 is substantiallydetermined by the amount of torque to be transferred by the CVU. Thepressure level at the pulley 20 is determined by the ratio required toprovide the commanded vehicle speed. The present invention insures thatthe pressure ratio between the pulleys 20 and 22 will be at the requiredproportion to establish the desired drive ratio in the CVU withoutaffecting the system pressure requirement at the pulley 22. It should benoted that the ratio of the control chambers of the sheaves 26 and 32 issuch that when the pwm control solenoid 50 is operating at a duty cycleof one hundred percent, the CVU ratio will be a two to one overdriveratio.

The schematic representation shown in FIG. 2 includes the powertrain 10including the engine 12, the gear set 14, the differential 18 and theCVU 16 comprised of the pulleys 20 and 22 and the belt 24. A controlsystem 38A has pump and control 40A which supplies line pressure to amain passage 36A. The main passage 36A is connected with secondarypulley signal valve 60, a primary pulley control valve 62 and a pressurelimit valve 64. The valve 60 is connected to the control 40A through abias signal passage 66 which transmits a pressure signal proportional tothe input torque to the CVU 16. The valve 60, which is essentially adownstream regulator valve, regulates the pressure in a secondary pulleypassage 68 in accordance with the pressure in the passage 36A. Thepressure is passage 68 is proportional to but less than the pressure inpassage 36A.

The secondary pulley passage is in fluid communication with the variablesheave 32, a ball shuttle valve 70, and a pwm control valve 72. The ballshuttle valve 70 communicates with the pressure limit valve 64. The pwmcontrol valve 72 communicates a control pressure to a control passage 74which is connected with a control area, not shown, on the control valve62. As described above for the valve 46, the pwm control valve 72receives a duty cycle signal through a wire 52A from the control 40A.The valve 72 then outputs a pressure signal to the passage 74proportional to the input pressure of the valve 72, which is thepressure in the secondary pulley passage 68.

The primary pulley control valve 62 provides a control pressure to thevariable sheave 26 of the pulley 20 through a passage 76. The passage 76is also connected with the shuttle valve 70. As is well known, the ballshuttle valve 70 will supply the highest of the pressures in thepassages 76 and 68 to the system pressure limit valve 64 such that thesystem pressure cannot exceed the higher of these two pressures.

The pressure in passage 76 is also communicated to a control port 78 onthe control valve 62. The port 78 supplies a control area, not shown, inthe valve 62 with a pressure equal to the control pressure in theprimary pulley 20. The area pressurized by the pressure in passage 76 isequal to one-half the area pressurized by the fluid in the controlpassage 74. Thus, the output pressure of the valve 62 in the passage 76will be equal to twice the pressure in the passage 74.

When the pwm solenoid is operated at a fifty percent duty cycle, thepressure in the passage 74 will be equal to one-half the pressure in thepassage 68. However, the pressure in the passage 76 will be equal to thepressure in the passage 68 such that the control pressures at thevariable sheaves 32 and 26 will be equal and the ratio of the CVU willbe one to one. As the duty cycle at the pwm increases, the pressure atthe variable sheave 26 will increase relative to the pressure in thepassage 68. This will place the CVU 16 in overdrive ratios when the dutycycle is between 50% and 100%. This will permit the pressure chambers ofthe pulleys 20 and 22 to be equal as compared to the system described inFIG. 1, where the pressure chamber of the pulley 20 is twice the area ofthe pressure chamber of the pulley 22.

What is claimed is:
 1. A control for establishing a drive ratio betweena primary pulley and a secondary pulley in a continuously variable unit,said control comprising:a source of pressurized hydraulic fluid; passagemeans for communicating a first pressure level of pressurized hydraulicfluid to the secondary pulley, a first pressure control valve and asecond pressure control valve; said first pressure control valvecommunicating a control pressure at a second pressure level ofpressurized fluid proportional to said first pressure level to saidsecond pressure control valve; and said second control valvecommunicating an operating pressure at a third pressure levelproportional to said first pressure level to said primary pulley forcontrolling a drive ratio between said primary and secondary pulleysproportional to the ratio of said first pressure level to said thirdpressure level.
 2. A transmission and control comprising:a continuouslyvariable unit having a first adjustable pulley and a second adjustablepulley; an electro-hydraulic control having a source of fluid pressureat a first pressure level; passage means communicating fluid pressure atsaid first pressure level to said first adjustable pulley, a controlvalve and a regulator valve; said control valve issuing a bias controlpressure to said regulator valve; and said regulator valve issuing apressurized fluid to said second pulley at a second pressure levelresponsive to said bias control pressure and being proportional to saidfirst pressure level to control a ratio between said first and secondpulleys at a value substantially equal to the ratio of said firstpressure level of pressure to said second pressure level.
 3. Thetransmission and control defined in claim 2 further comprising:said biascontrol pressure being at a level substantially equal to said secondpressure level.
 4. A control for establishing drive ratios between aprimary pulley and a secondary pulley in a continuously variable unit,said control comprising:a source of pressurized hydraulic fluid; passagemeans communicating with said source for conducting a first pressurelevel of pressurized hydraulic fluid to the secondary pulley, to a firstpressure control valve and to a second pressure control valve; saidfirst pressure control valve communicating a control pressure at asecond pressure level of pressurized fluid proportional to said firstpressure level to said second pressure control valve; and said secondpressure control valve communicating an operating pressure at a thirdpressure level proportional to said first pressure level to said primarypulley for controlling the drive ratio between said primary andsecondary pulleys.
 5. The transmission and control defined in claim 4further comprising:said source of pressurized fluid comprising a thirdpressure control valve means for reducing said pressurized fluid priorto conduction to the first adjustable pulley and said first pressurecontrol valve.
 6. The transmission and control defined in claim 5wherein said second pressure control valve presents a control pressurelevel to the second adjustable pulley that is equal to a controlpressure at the first adjustable pulley when the ratio between thepulleys is one to one.
 7. The transmission and control defined in claim4 wherein said second pressure control valve presents a control pressurelevel to the second adjustable pulley that is equal to a controlpressure at the first adjustable pulley when the ratio between thepulleys is at a maximum overdrive ratio.